1. Field of the Invention
The present invention relates to phase shifters for propagating electromagnetic energy. More particularly, the invention relates to a low-loss, compact two-bit phase shifter suitable for use in aeronautical beam steering antennas, phase shift keying (PSK) data communication systems, and other applications.
2. Description of the Related Art
The use of antennas on mobile platforms has grown dramatically with an increased demand by users to stay in touch in a more mobile society. This increased demand spans bidirectional exchange of data using mobile platforms for both personal and business needs. To meet this need, the moving platform, such as an automobile, a news reporting van, a boat or an airplane, typically uses an antenna that is able to track, or “lock onto” a signal source, such as a satellite or a stationary terrestrial base station or broadcast tower. In particular, phased array antennas with beam steering functionality often are used to provide this capability.
These antennas typically use a number of phase shifters to vary the phase of radio frequency (RF) signals in a coordinated manner across the radiating elements of the antenna array to point or steer the beam of the antenna in a desired direction. This type of beam steering can be used to track or lock onto a target regardless of the movement of the platform to which the antenna is attached. These phase shifter array antennas are usually bidirectional in that the beam of the antenna can be pointed to a target, such as a satellite, to both receive signals from and send signals to the satellite or another component in the communication system. In other words, a phase shifter in a reciprocal antenna can facilitate full duplex communications in a mobile communication system.
In the case of phased array antennas mounted on airplanes, referred to as aeronautical antennas, a number of design factors become important beyond the beam steering capability of the antenna. One of these design factors involves the phase shifter component itself. The phase shifter should be as small as possible, thus reducing the amount of space on a circuit board onto which it is mounted with other antenna components. For example, to minimize its size it is desirable for the circuit to have a minimum number of control lines. Also, the phase shifter should have insertion loss as low as possible. These and other design considerations are sometimes in conflict, making different configurations preferable for different applications depending on the importance of the various design consideration for the particular application.
Phase shifters suitable for the applications described above are often connected in a series of stages, with a first coarse phase shifter followed by a second fine tuned phase shifter, to deliver the required phase shift to each antenna element. Conventional phase shifters are typically configured as switched-line or one-bit phase shifters. One-bit phase shifters typically shift the phase of an input signal between a first state (usually 0.degree. or reference phase shift) and a second state (e.g., 90° or 180° phase shift). See, for example, Nakada, U.S. Pat. No. 6,542,051, which shows a number of designs for one-bit phase shifters for digitally shifting the phase of a radio frequency (RF) signal by changing a switched line that is connected between the input and output main lines.
These and other conventional one-bit phase shifters typically include switched line phase shifters, which may be composed of two main lines, two or more switched lines (e.g., a reference line and one or more delay lines), and a plurality of radio frequency (RF) switches. Each end of a switched line is connected to one of the main lines, typically through an RF switch. When one of the switched lines is connected between the two main lines via the appropriate switches, a phase shift occurs in an RF signal that passes through the phase shifter. The amount of the phase shift depends on the length of the switched line and the corresponding amount of signal delay caused by the switched line.
Referring now to FIG. 1A, this drawing corresponds to FIG. 2 from the Nakada patent which illustrates a simplified schematic diagram of a conventional phase shifter 10. The phase shifter 10 includes a main input line 12, a main output line 14, a first or reference switched line 16, a second or delay switched line 18, and a plurality of switches 22, 24, 26 and 28. As shown, the reference switched line 16 is connected between the main input line 12 and the main output line 14 through switches 22 and 24, and the delay switched line 18 is connected between the main input line 12 and the main output line 14 through switches 26 and 28. The electrical length of the delay switched line 18 is longer than that of the reference switched line 16.
In operation, the switches 22, 24, 26 and 28 operate together to connect either the reference switched line 16 or the delay switched line 18 between the main input line 12 and the main output line 14. That is, when the reference switched line 16 is to be connected between main input line 12 and the main output line 14, the switches 22 and 24 are closed or “ON” and the switches 26 and 28 are open or “OFF.” Similarly, when the delay switched line 18 is to be connected between the main input line 12 and the main output line 14, the switches 22 and 24 are open and the switches 26 and 28 are closed. By switching the signal path from the main input line 12 to the main output line 14 through either the reference switched line 16 or the delay switched line 18, a phase shift is effected in an RF signal that passes through the phase shifter 10. The magnitude or amount of the phase shift corresponds to the electrical length difference between the reference switched line 16 and the delay switched line 18.
For example, the electrical lengths of the reference switched line 16 and the delay switched line 18 can be such that a phase shift of zero degrees (i.e., the reference delay, which is typically designated as zero degrees) occurs when the reference switched line 16 is connected between the main input line 12 and the main output line 14, and a phase shift of 90° (i.e., ninety degrees more than the reference delay) occurs when the delay switched line 18 is connected between the main input line 12 and the main output line 14. In such example, the length of the referenced switched line 16 is λ/4 (a quarter-wavelength where λ is the wavelength of the input signal) and the length of the delay switched line 18 is a half-wavelength, λ/2. The quarter-wavelength difference in electrical length between two switched lines (i.e., a half-wavelength minus a quarter-wavelength) causes a phase shift of ninety degrees (90°) in the input RF signal. However, it should be noted that two switches are present in the signal path for each states of this particular one-bit phase shifter.
FIG. 1B is a simplified schematic diagram of another conventional phase shifter 30, which has a slightly different configuration, as shown in FIG. 9 of Nakada. The configuration of this phase shifter 30 is similar to that of the phase shifter 10 in FIG. 1A except that the delay switched line 18 is connected directly to the main output line 14. That is, the delay switched line 18 is connected to the main output line 14 without a switch, such as the switch 28 shown in FIG. 1A. In this arrangement, the delay switched line 18 will always be connected to a main line, even when the reference switched line 16 is connected between the main input line 12 and the main output line 14 (i.e., when the switches 22 and 24 are closed and the switch 26 is open). The constant connection between the delay switched line 18 and the main output line 14 is beneficial to the overall operation of the phase shifter 30. For example, such arrangement reduces phase shift deviation, which, in general, involves the deviation of the phase shift when the frequency of an input RF signal varies. Nevertheless, two switches are present in the signal path in one of the states of the one-bit phase shifter shown in FIG. 9 of the Nakada patent.
Two-bit phase shifters typically shift the phase of an input signal between one of three or four states, e.g., zero degrees, ninety degrees, one hundred eighty degrees and two hundred seventy degrees (0°, 90°, 180° and 270°). To provide two-bit (i.e., up to four state) phase shift functionality, two one-bit phase shifters are typically cascaded in series. This arrangement takes up a relatively large amount of space on a circuit board. This configuration also requires a relatively large number of switches including bypass and cascade switches as well as up to four switches for each one-bit phase shifter. This configuration also experiences relatively large signal insertion loss because the signal passes through at least two switches in each state.
As a result, there continues to be a need for a compact, low-loss two-bit phase shifter. In particular, there is a need for a two-bit phase shifter that has fewer components, a smaller size, and a simpler structure than conventional two-bit phase shifters. There is a further need for a two-bit phases shifter with lower insertion loss than conventional two-bit phases shifters.